1. Field of the Invention
The invention relates to the field of semiconductor assembly or packaging, and more particularly to lead frame structures and methods for manufacturing the same.
2. Description of the Related Art
Conventional lead frame structures used in semiconductor packaging processes include the following two types. One type is a Quad Flat No-Lead (QFN) metal lead frame. For this type, as shown in FIG. 11, to prevent molding compound overflow from the back surface of the lead frame during an encapsulation process on the top surface of the lead frame, a layer of expensive high-temperature resistant film 23 is affixed on the back surface of the lead frame. Such a lead frame may have certain disadvantages:
1) Because an high-temperature film is affixed on the back surface of the metal lead frame, the cost of lead frame is substantially increased (e.g., at least 50%);
2) The adhesive film affixed on the back surface of the metal lead frame often is a soft organic material. During subsequent assembly and wire bonding processes, high-temperature baking may cause organic volatile contamination, which may have direct impact on the bonding characteristics between the wires and the top surface of the chip and between the wires and the top surface of the lead frame. It may even cause bonding failure (so-called delamination) between the molding compound and the top surfaces of the chip and lead frames in later packaging processes;
3) In the subsequent wire bonding process, because the back surface of the lead frame is affixed with the soft organic adhesive film, part of the bonding strength may be absorbed by the soft organic adhesive film. This may increase the difficulty for wire bonding, cause unstable wire bonding yield, and may cause reliability issues;
4) Because the back surface of the lead frame is affixed with the soft organic adhesive film, the metal wire material used in the wire bonding process may be limited to relatively softer and more expensive gold wires. Other relatively harder but less expensive materials, such as copper, aluminum, or other metal wires or strips, may be undesirable; and
5) In the subsequent molding process, because the back surface of the lead frame is affixed with the soft organic adhesive film, the molding pressure during the molding process may cause certain mold bleeding between the lead frame and the adhesive film, which may put bleeding material between the lead pad 22 or die pad 21 and the organic adhesive film, as shown in FIGS. 12 and 13 (molding compound under certain metal leads and die pad on the left side of the drawing).
The other type is a two-side etching and pre-encapsulated lead frame, as shown in FIG. 14. To form this type of lead frame, an etching process is first performed on the back surface of the metal substrate, and the etched areas on the back surface are sealed with encapsulation material. Further, etching and plating processes for inner leads are performed on the top surface of the metal substrate. However, such a lead frame structure may have following disadvantages:
1) Because the manufacturing process of the lead frame is complicated, manufacturing cost may be increased;
2) Because the lead frame needs to be etched from the top surface and the back surface separately, it may likely cause dislocation due to re-positioning errors when setting the top surface and back surface etching positions.